Invaders! What makes a species invasive?

What Makes a Plant Species Invasive?

Botanists, ecologists, and biologists have asked this question for decades. What may seem simple isn’t actually all that clear.

In the animal kingdom, invasive species are relatively easy to identify. The Burmese python, released in the Everglades, quickly demonstrates its invasive qualities as it slithers its way to the top of the food chain wherever it becomes established.

But for the careless weed that hitches a ride on an unsuspecting shipment bound for another country, the line between alien visitor and invasive threat becomes blurred.

Plants like Cogongrass (Imperata cylindrica) arrived in the United States as packing material for cargo. Dried out, the plant wasn’t viewed as a potential problem. Florida’s warm climate fostered its growth, and today it prevails throughout much of the Southeast, particularly in Florida, Alabama, Mississippi, Louisiana, and Georgia.

Why is it problematic? This aggressive monoculture outcompetes native grasses. Its greatest advantage is a massive network of rhizomes (underground stems). A single patch can expand outward every year, and even small rhizome fragments left in the soil can establish new colonies.

Another problematic plant is hydrilla. Introduced through the aquarium trade in the 1950s, it was originally purchased for fish tanks. Unfortunately, some hydrilla plants were released or escaped into Florida waterways. Once established, they spread rapidly through canals, rivers, and lakes, posing a significant threat to aquatic ecosystems.

Often, plants arrive on purpose. A clear example is kudzu, the vine currently choking out plant life throughout Tennessee and neighboring states. During the warm season, it can be seen sprawling across roadsides like a scene from Jumanji.

Native to East Asia, kudzu packs a painful punch. It was intentionally introduced to the United States and showcased at the 1876 Centennial Exposition as an attractive ornamental plant. Gardeners were captivated by its lush foliage and fragrant purple blooms, leading to its widespread adoption in yards and gardens. It was even promoted as a solution for soil erosion.

What wasn’t fully understood was how aggressively kudzu would spread in the warm, humid Southeast. Free from many of the insects, diseases, and grazing pressures that kept it in check in its native range, kudzu escaped cultivation and spread rapidly across forests, roadsides, and utility corridors.

Now let’s discuss the plants that give people pause.

The beautiful mimosa tree is almost always a topic of debate. Its list of drawbacks is long, yet its advantages cannot be ignored. Gracing lawns with striking pink blooms, these pollinator-friendly trees are, in many ways, a beautiful tragedy.

Introduced from Asia for their undeniable charm, mimosa trees produce bright, fluffy flowers that eventually become countless seeds. Those seeds spread easily into roadsides, woodlands, and unsuspecting neighboring yards. Ecologists and botanists generally agree that mimosa trees can negatively impact native ecosystems. However, the abundance of beneficial insects and pollinators that visit their blooms keeps the debate alive.

Another frequently debated invasive species is the Bradford pear. At one time, this tree was highly sought after for its showy white flowers and ease of cultivation. It seemed like the ideal ornamental tree because it was believed to be sterile.

The problem arose when different cultivars were planted near one another, allowing cross-pollination to occur. Eventually, those previously “sterile” trees began producing viable seeds that birds dispersed across the landscape.

Today, many states discourage or even prohibit the sale of Bradford pears.

As landscapers and plant nurseries seek new and exciting additions for beautiful garden spaces, the conversation surrounding invasive plants continues to spread like seeds on the wind.

So what can be done?

Smart native swaps are almost always the answer. Mimosa trees can be replaced with the native Eastern redbud for a similar burst of spring color. Native honeysuckles and passionflowers can help create a lush, tropical feel without the ecological consequences of kudzu.

In aquatic settings, native pondweeds can provide habitat and beauty without the dense, ecosystem-choking growth associated with hydrilla.

In light of the growing threats facing our environment, understanding how plant species are introduced—whether intentionally or accidentally—is critical to protecting native ecosystems. The choices we make in our gardens, landscapes, and waterways today can determine what our natural spaces look like for generations to come.